1. Field Of The Invention
The present invention relates to a method for reproducing holograms, kinoforms, diffractive optical elements, microstructures, and a plastic binary optical element preferably produced by such a method. In particular, the present invention relates to a method for mass producing plastic elements containing functional (holograms, microgears, etc.) and non-functional (decorative) surface irregularities or discontinuities by the process of plastic molding (injection molding, casting, etc.). The present invention also particularly pertains to a plastic molded binary optical element capable of performing optical functions which heretofore required a plurality of lenses and/or other optical structures.
2. Description Of The Related Art
Previous methods and techniques developed for casting and injection molding of plastic microstructures have proven to be either inaccurate, too expensive, or too time consuming to be used for mass production, such as U.S. Pat. No. 5,227,897 to Fohrman, et al. For example, U.S. Pat. No. 5,071,597 to D'Amato, et al., discloses a technique for forming a mold to replicate large numbers of plastic articles by injection molding. The mold contains a hologram or other microstructure to be transferred to an outside surface of the molded article. First, a model of the article to be molded (e.g., a bottle cap) is prepared having a hologram on one surface thereof. The hologram is prepared by exposing a photosensitive film to two beams of coherent light that intersect each other at the photosensitive film with a finite angle therebetween. As a result, the photosensitive film records an interference pattern between the two beams over the film's two-dimensional surface. If one of the light beams is modulated, a holographic microstructure is produced, whereas if neither beam is modulated, a simple diffraction grating is formed on the photosensitive film. The photosensitive film is then attached to the article model. After the model has been so-constructed, a thin layer of silver is electrodeposited over the hologram microstructure so that the silver faithfully follows the surface relief pattern. A nickel metal layer is then electrodeposited over the silver layer. Thereafter, the deposited nickel layer is removed from the model structure and attached to a backing plate for strength. The backing plate is then positioned in an injection molding cavity and liquid plastic is injected therein and allowed to cure or solidify to produce a plastic element having the hologram or other microstructure embedded therein.
However, it can been seen that this process is quite expensive and time consuming, requiring several electrodeposition steps to form the molding plate. Furthermore, the molding plate produced by such a method is usually quite thin and may become warped when it is inserted into the molding machine, or within the molding machine itself due to the high temperatures and pressures used in the injection molding process.
Other techniques for replicating a microstructure in a molding plate utilize methods such as vacuum deposition, chemical deposition, etc. Such techniques also result in warpage of the molding plate due to the thin nature of the insert, and require an excessive amount of time. Furthermore, the soft nature of the metals used in such techniques leads to premature wear of the molding plate, requiring additional costly and time-consuming replications of the molding plate.
U.S. Pat. No. 5,013,494 to Kubo, et al., discloses a process for preparing blazed holograms wherein the first generation hologram master itself is placed into the injection molding machine. In detail, a photoresist is coated on a glass substrate, subjected to exposure, and developed, thereby forming a predetermined photoresist pattern. Then, an ion beam etching treatment is effected on the surface of the glass substrate by using said pattern as a photomask, thereby providing an image transfer layer. The glass molding plate is inserted into the injection molding machine and an acrylic resin is injected therein to produce a plastic part having the microstructure embedded therein. Again, such a technique is costly, slow, and produces a fragile molding plate subject to breakage and/or premature wear.
In an alternative embodiment, Kubo, et al., discloses a process wherein a thin, 500-2000 .ANG. metal layer is deposited on an etched glass layer and then backed with an electro-deposited metal layer such as Ni, Ni--Co alloys, which is 0.2-0.4 mm thick. This metal layer is then removed from the glass and inserted into the injection molding machine. There are, however, a number of disadvantages to this process. First, the photoresist and the glass substrate will etch at different rates. This will cause a change in the geometry of the pattern as it is transferred into the substrate material. For example, if the photoresist pattern has peaks that are 1 micron above the substrate surface, and valleys that are 0.1 micron above the surface of the substrate, and if it is assumed that the resist etches at twice the rate of the glass substrate, then in the time it requires to etch through to transfer the 1 micron peaks into the glass, in the valley area, 0.1 micron of the resist will have been etched through and also 0.45 micron of the glass substrate. Therefore, what was a 0.9 micron high feature will now be only 0.45 microns in height. The geometry across the grating will not change, however. Second, if any of the structure in the photoresist shadows any lower structure (in the direction of the etch), accurate replication of the surface discontinuities will be impossible. In fact, Kubo, et al., describes a slantwise irradiation which can be used to create a blazed structure, but which would be unsuitable to creating holographic microstructures. Furthermore, the Kubo metal substrate is replicated from a glass substrate and is, therefore, a generation removed in accuracy. Again, such steps are time-consuming and expensive.
While certain U.S. patents speak of "casting a hologram" (U.S. Pat. Nos. 4,933,120; 5,003,915; 5,083,850; and 5,116,548), the liquid resin is actually cast onto a flexible paper web, and a hologram master is then pressed into the resin to form the surface relief pattern. Such a technique is not applicable to the mass production of plastic elements.
Binary optics is an emerging technology wherein an optical element (e.g., a lens) includes a surface relief pattern for effecting modulation of an optical wavefront passing therethrough. Thus, not only is the light beam refracted by the lens element, but it is also diffracted by the surface relief pattern to produce an image from a single incident plane wave. A binary optical element (BOE) may be defined as a diffractive optical element having multi-levels of phase which is a stepped approximation of an ideal surface profile of a kinoform lens (see FIG. 1C). The kinoform lens, like a Fresnel lens, has a discontinuous thickness (or refractive index) profile. However, the operation of a kinoform relies on its interference of light from different zones, i.e., diffraction mechanisms (optical path difference at the discontinuities is an integral number of wavelength), while a Fresnel lens bends rays of light by the refraction mechanism (optical path difference at the discontinuities is not carefully controlled). Kinoforms are also called "micro-Fresnel lenses". The performance of the kinoform can be diffraction-limited, but that of the Fresnel lens is not diffraction-limited.
FIG. 1A is a schematic depiction of a quadratic kinoform showing a corresponding conventional lens in dashed line. A corresponding linear kinoform of slightly lower efficiency is depicted in FIG. 1B. The corresponding BOE is depicted in FIG. 1C, which may have higher or lower efficiency than the linear kinoform depending on the number of levels used. Binary optics refers to the dual-level (high-low) nature of the phase-relief pattern used to control the phase, amplitude, and polarization of an optical wavefront. FIG. 1C depicts a four-level relief structure which may be fabricated using the same technologies used to produce VLSI devices in the electronics industry.
The properties of BOE's can be exploited to carry out a variety of tasks such as dispersion compensation, thermal compensation, beam steering, optical multiplexing, light wave modulation, optical interconnecting of a variety of light signals, collimating, light wave redistributing, etc. These different functions may be achieved by varying the location and size of the array of phase gratings on the surface of the lens.
Heretofore, the production of BOE's has been costly and time consuming because such BOE's are produced by painstakingly etching the individual binary microstructure onto the surface of a polished glass lens or mirror. Alternatively, a BOE may be produced by providing a conventional glass lens with a coating of photoresist with a holographic structure individually patterned into the resist layer. See U.S. Pat. Nos. 4,895,790; 5,161,059; and 5,218,471 for such conventional techniques. However, these techniques require a great amount of labor, capital and time, as with the other techniques discussed above. However, binary optics are advantageous in that one or a small number of BOE's may replace lens systems requiring a significant number of glass elements, such as wide field of view systems.
Accordingly, what is needed is an inexpensive, accurate, and fast method for producing durable plastic elements having a microstructure embedded therein, and a plastic BOE produced by such a method.